In a remarkable development, researchers have discovered that over half of the world’s oceans have undergone a significant greening effect in the past two decades, primarily due to the impact of global warming. This groundbreaking finding, reported today in the prestigious journal Nature, has taken scientists by surprise, as they initially believed it would take many more years of data to detect such signs of climate change through alterations in ocean color.
The lead author of the study, B. B. Cael, an ocean and climate scientist at the National Oceanography Centre in Southampton, UK, emphasized the significance of the findings, stating, “We are affecting the ecosystem in a way that we haven’t seen before.”
The color of the ocean can undergo changes for various reasons, such as the upwelling of nutrients from its depths, leading to substantial phytoplankton blooms that contain the green pigment chlorophyll. By studying the wavelengths of sunlight reflected off the ocean’s surface, scientists can estimate the amount of chlorophyll and thereby assess the abundance of living organisms like phytoplankton and algae. With climate change causing ocean waters to warm, it theorized that biological productivity would impacted.
However, the variability of chlorophyll in surface waters from year to year made it difficult to discern climate change-induced changes from natural fluctuations. Scientists believed it would require up to four decades of observations to identify any discernible trends. Additionally, data collected different satellites measuring ocean color using slightly different methods couldn’t effectively combined.
To address these challenges, Cael’s team focused on analyzing data from MODIS, a sensor aboard NASA’s Aqua satellite, launched in 2002 and still operational beyond its anticipated lifespan. Rather than relying on a single wavelength to track chlorophyll, they examined trends across seven different wavelengths of light. This comprehensive approach allowed them to detect long-term shifts in ocean color using two decades of MODIS data.
The study revealed significant changes in the color of 56% of the world’s ocean surface, predominantly in the waters between latitudes 40º S and 40º N. These tropical and subtropical regions, characterized by relatively stable colors throughout the year, exhibited more noticeable long-term alterations, according to Cael.
To determine if these shifts were linked to climate change, the researchers compared the observations with a model simulating how marine ecosystems might respond to increasing greenhouse gas levels. The observed changes closely aligned with the model’s predictions, strengthening the connection between ocean greening and climate change.
Ocean Experiencing Color Changes
As scientists delve into the causes behind the ocean’s greening, it appears that the transformation is not directly correlated to rising sea surface temperatures. Interestingly, the areas experiencing color changes do not consistently align with regions of temperature increase. Instead, researchers hypothesize that alterations in nutrient distribution within the ocean play a role. Warmer surface waters lead to greater stratification, making it more challenging for nutrients to rise. This change could favor smaller phytoplankton species over larger ones, ultimately impacting the ecosystem and resulting in visible alterations in water color.
Nevertheless, the exact reasons for the observed greening phenomenon remain under investigation. Cael emphasized the importance of understanding the ecological implications reflected in the changing color of the oceans.
This remarkable discovery amplifies the anticipation surrounding the upcoming launch of NASA’s Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite in January 2024. Equipped with hyperspectral capabilities, PACE will measure ocean color across an extensive range of wavelengths, surpassing the capabilities of any previous satellite. Scientists anticipate that PACE will provide valuable insights into the ecological consequences of the observed trends in ocean ecosystem structure for years to come.
Ivona Cetinić, an oceanographer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, involved in the PACE mission, acknowledged the significance of the findings and expressed the necessity for global hyperspectral missions like PACE to deepen our understanding of these transformations.
The greening of the oceans serves as a potent visual reminder of the far-reaching effects of climate change and underscores the urgent need for further research and action to mitigate its impact on marine ecosystems worldwide.
